Phone systems for businesses have typically provided a higher level of functionality than basic telephone service, providing functions such as blind call transfer, attended call transfer, line and set availability indication, to name but a few. This functionality has classically been provided by some type of central control, most commonly a Private Branch Exchange (PBX) or more recently “telephone server”.
The central control systems have drawbacks in terms of high initial cost, single point of failure and complexity in setup and administration. The initial cost of the central control point was particularly prohibitive to small businesses that only needed a few telephones.
To address these limitations some phone systems have been developed that don't have a central control. An example of a telephone system without a central control point is U.S. Pat. No. 4,757,496, titled “Distributed Telephone System”, which describes a telephone system where each Telephone Set is coupled to a common coaxial cable. Unfortunately, this system still requires either a fixed administration, or manual programming, to set some system parameters such as Telephone Set addresses. More importantly it was limited in that the phones had to be geographically located relatively close together.
U.S. Pat. No. 6,256,319, titled “Plug and Play Telephone System” also describes a telephone system without a central control point. It describes a system where each Telephone Set is connected to every other Telephone Sets by a common telephone line connection. Across this common telephone line connection a radio frequency (RF) based communication channel is created. Unfortunately, this RF communication channel method imposes numerous limitations on the telephone system operation. Notably, these include:                limits to the number of telephone sets that can share the common telephone line connection because of regulatory limitations on “ringer equivalence number” (REN) loading factors. This factor limits how large the telephone system can become, typically up to only 12 telephone sets.        the RF communication channel method is limited by telephone line lengths, terminations and interference with common DSL services. This factor limits the reliability and performance of the system in real-world deployments.        the RF communication channel method only allows the telephone system to operate from a single user premise. This factor prevents the system from operating across a geographically distributed area.        the RF communication channel method only allows a limited number of simultaneous full-duplex audio paths across the communication channel. This factor limits system functionality and size, because users and/or features are blocked when all of the full-duplex audio paths are in use. The more users in a system, the greater the likelihood that blocking would occur.        the RF communication channel method, as commonly implemented, requires that multiple analog phone lines are cabled to each phone location. This factor increases the cost and complexity of wiring up the phone system in real-world deployments.        
With the advent of Voice-over-data or Voice-over-Internet-Protocol (VOIP), business oriented telephone systems have been created that use VOIP telephone sets and VOIP aware central point of control commonly called an “IP PBX” or simply “telephone server”. This central control point provided functionality similar to classical PBXs and had the same drawbacks of single point of failure and high initial cost relative to deployments of small numbers of phone sets.
While individual VOIP phones could call each other without the use of a IP PBX, this is more analogous to basic telephone service without such features as set-availability or line-in-use indicators.